1. Field of the Invention
This invention relates to naturally commutated static power converters and, more particularly, to a method and means for reducing the circulating current in the positive and negative banks of such converters.
2. Prior Art
Cycloconverters are a class of static power frequency changers which generate an electrical output waveform having a frequency which differs from the input frequency. Some cycloconverters generate an output waveform of fixed frequency from an input waveform of variable frequency. Such systems are used for instance in airborne electric power generating systems where the generator is driven by the aircraft engine which operates at various RPM settings while a constant frequency AC voltage is required to operate the aircraft electrical equipment. Another type of cycloconverter generates a variable frequency output signal from a fixed frequency input. Such converters are often used in controlling the speed of AC motors. The term static power converters as used herein is also intended to cover dual converters which generate a DC output of either polarity and are often used in controlling DC motors. For a more thorough understanding of the various types of static power converters, see Static Power Frequency Changers, L. Cyugyi and B. R. Pelly, Wiley-Interscience 1976.
The converters to which the invention applies utilize naturally commutated thyristor power circuits. The thyristor (also known as a silicon controlled rectifier or SCR) is a static power switch which, when properly biased, conducts current in a single direction in response to a firing pulse applied to a gate electrode and continues to conduct until the current flow is terminated for a preset interval. In a naturally commutated static power converter, the firing pulses are generated at instants such that the phase voltage of the thyristor which is turned on is sufficient to commutate off the next preceding thyristor. Thus no additional circuitry is required to turn off the thyristors, and they are said to be naturally commutated.
The thyristors of the naturally commutated static power converter are arranged in positive and negative banks which conduct the positive and negative portions of the output current respectively. While the mean value or fundamental component of the output voltage waveforms of the positive and negative thyristor banks are equal, the instantaneous values are not since they are derived from different portions of the source voltage waveforms. Therefore, if the thyristors of each bank are continuously fired, a circulating current will flow between the banks. This portion of the circulating current is referred to as the ripple component and is developed in both the dual converter and the cycloconverter. When the fundamental output current of the converter varies, as it quite clearly does in the case of the cycloconverter which, by definition, generates an AC output and as it may do to a small extent in a dual converter under actual operating conditions, an additional component of the circulating current called the "self-induced" component is generated. For a more complete explanation of these circulating current components and how they are developed see Thyristor Phase-Controlled Converters & Cycloconverters, B. R. Pelly, Wiley-Interscience 1971, pages 126-144 and 151-160. As explained therein, the theoretical average value of the "self-induced" circulating current in a cycloconverter is 0.57 times the average output load current. Clearly, this imposes a substantial "wattless" load on the converter and the voltage source, in addition to the "useful" load.
One solution to the above problem is to eliminate the circulating current entirely by a technique known as bank selection in which firing pulses are withheld from the nonload carrying bank. Examples of bank selector schemes are disclosed in U.S. Pat. Nos. 3,568,033 and 3,852,654 and the commonly owned copending application of Stacey, et al., Ser. No. 95,899 filed concurrently herewith. Bank selection is also discussed in the above mentioned books. While bank selection has proved useful in many applications, its use results in unacceptable distortion in the output waveform in applications where there are sizable discontinuities in the output current. Under such circumstances continuous firing of the thyristors in both banks with the attendant circulating current is the only way to achieve an output waveform of suitable quality. In order to reduce the additional load imposed on the system, however, attempts have been made to lower the circulating current such as by adding a DC bias to the firing pulse generating circuit to adjust the firing instants of the banks. However, since this bias affects generation of firing pulses for the load carrying thyristor bank as well as the nonload carrying bank, this arrangement causes considerable distortion in the output waveform. To reduce this distortion, and overall feedback signal from the output to the two bank firing angle control circuits can be used; however, since the feedback can only correct the firing instants of the load supporting bank after it has begun to supply output current and since its effect cannot be instantaneous, some increased distortion will still exist. An example of this type of control is disclosed in U.S. Pat. No. 3,593,106.
A compromise solution described in Thyristor Phase-Controlled Converters & Cycloconverters, at pages 190 to 198, provides for simultaneous firing of both banks and therefore the presence of circulating current when the load current is below a certain threshold, and for removing the firing pulses from the nonload carrying bank and thereby eliminating the circulating current when the instantaneous load current is above the threshold level. While this arrangement reduces the peak value of the circulating current appreciably, it introduces its own distortion into the output waveform. The control system disclosed in U.S. Pat. No. 3,568,033 mentioned above utilizes this controlled pulse overlap technique.
It is also known to provide end stop limits for the firing instants of the thyristors in cycloconverters to maintain conditions for natural commutation. A rectification end stop prevents firing of a thyristor too soon and an inversion end stop forces the firing of a thyristor if the control circuit has not produced a firing pulse by a given instant. An example of an end stop control is disclosed in U.S. Pat. No. 3,818,315. The commonly owned copending application of Stacey, Ser. No. 95,803 filed concurrently herewith discloses an end stop control which produces adjustable end stops which not only maintain natural commutation in the converter but also permit the converter to accommodate for load faults. End stop controls are also discussed in Static Power Frequency Changers at pages 308-311 and in Thyristor Phase-Controlled Converters & Cycloconverters at pages 259-271. The end stop controls of the prior art are applied to both the load carrying and nonload carrying banks of the converter.
Taking into account the limitations of the prior art discussed above, it is a primary object of the present invention to provide an improved, naturally commutated static power converter and a method of operating the same that produce a quality output waveform without imposing a sizable wattless load on the multiphase AC source.
It is also an object of the invention to realize the above object by controlling the interbank circulating current.
It is another object of the invention to control the interbank circulating current by retarding the firing pulses to the nonload carrying bank only.
It is yet another object of the invention to achieve the above objects with a minimum of complexity and hardware required.